Annals of Botany

Genome size and growth rates — grass goes against the trend

As biologists we hold two contrasting ideas. The first is that genomes are stable and that organisms possess mechanisms to ensure that stability. The second is that genomes are dynamic and have clearly changed over evolutionary time. Plants are one of the groups that exhibit the latter especially clearly: closely related species, even within the same genus, may possess genomes of very different sizes, based particularly on variation in the amount of non-coding DNA. If such changes occur over evolutionary time, might we not expect to observe in the present time, ‘changes in progress’, such as subtle differences in genome size between individuals that may represent an early stage of divergence? The answer, at least for the grass Festuca pallens, is clearly Yes. Šmarda et al. (Brno, Czech Republic, pp. 599–607) have used flow cytometry to measure DNA amounts in 562 F. pallens seedlings derived from 17 maternal plants from a single population. To eliminate artefactual differences resulting from experimental error, they checked by co-processing in pairs plants whose genome sizes had been shown to differ. Only if this resulted in a double peak in the flow cytometer were the differences regarded as genuine. The data thus verified were very clear. Amongst all the seedlings there was a 1.188-fold variation in genome size; even for the progeny of one maternal plant the variation was 1.119-fold. There was, however, also a tendency to the mean, probably caused by the variation in the genome sizes of the paternal plants pollinating an individual maternal plant. Nevertheless, the data also indicated a possible selective advantage of larger genomes, contrasting with the accepted view that larger genomes are associated with slower growth rates. The authors found a positive correlation between growth rate and genome size, possibly explaining why this population in its particular habitat had a higher mean genome size than a nearby population.